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Article

Electrode Material Optimization of Nitrous Oxide Recovery from Incineration Leachate in a ΔnosZ Pseudomonas aeruginosa/MEC System

by
Yong Liu
1,
Bing Yan
1,
Song Xia
1,
Shuanglin Gui
1,
Haiwei Jiang
1,
Hanbing Nie
1,2,* and
Dezhi Sun
2
1
Institute of Energy, Jiangxi Academy of Science, Nanchang 330096, China
2
College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Fermentation 2023, 9(7), 607; https://doi.org/10.3390/fermentation9070607
Submission received: 19 May 2023 / Revised: 11 June 2023 / Accepted: 25 June 2023 / Published: 28 June 2023
(This article belongs to the Special Issue Progress in Microbial Treatment of Wastewater, Solid Wastes and Waste Gases)
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Abstract

:
Nitrous oxide (N2O) is not only recognized as a potent greenhouse gas, but it is also used in industry as a clean energy source. In this study, different electrode materials of carbon felt and graphite were equipped in the ΔnosZ P. aeruginosa/microbial electrolysis cell (MEC) systems to explore the optimization mechanism for long-term N2O recovery during incineration leachate treatment. The carbon felt group showed a better performance in N2O recovery across 45 days of operation. The N2O conversion efficiency was above 80% and the proportion of N2O in biogas accounted for 80.6% in the carbon felt group. qRT-PCR analysis was conducted to evaluate the expression of genes involved in denitrification (norB) and electroactivity (phzG, phzM, and phzH) of ΔnosZ P. aeruginosa. The results showed a significant upregulation in the suspended biomass (day 21) and the electron-attached biomass (day 45) from the carbon felt-equipped reactor, which was highly related to the opportunity of biomass exposed to the phenazine derivatives. By the carbon felt optimization in the system, 82.6% of the Pseudomonas genus survived after 45 days of operation. These results indicate that the carbon felt electrode has a more sustainable performance for N2O recovery in the ΔnosZ P. aeruginosa/MEC system.

1. Introduction

Incineration leachate is a type of wastewater that contains high concentrations of organic matter (COD, 30,000~70,000 mg/L) and nitrogen (mainly NH4+-N, 1000~2000 mg/L), with a complex composition [1]. Typically, it is treated using a composite process of “pretreatment-biological treatment-advanced treatment” [2]. Organic matter and ammonia in the incineration leachate are primarily removed through the “anaerobic digestion-aerobic nitrification-anoxic denitrification” process in the biochemical treatment section. With the continuous advancement of wastewater resource research, most organic matter in incineration leachate can be converted into energy through optimized anaerobic digestion to produce methane [3,4]. However, research on the recovery of nitrogen resources is relatively scarce.
In the commonly used nitrification–denitrification process for nitrogen removal, a portion of the ammonia is converted into a controversial intermediate product known as nitrous oxide (N2O). N2O is a recognized potent greenhouse gas, but it has a higher calorific value when burned with CH4 than O2 and is also a clean energy source [5]. In unregulated wastewater treatment, the conversion efficiency of N2O is only 0.11~1.90% [6,7,8,9,10], which cannot be recovered as energy and also causes the greenhouse effect. Therefore, by regulating the microbial metabolism involved in the nitrification–denitrification process, ammonia, nitrate, and nitrite in wastewater can be efficiently converted into N2O and then collected. This not only achieves energy recovery of N2O during wastewater denitrification treatment but also reduces greenhouse gas emissions from wastewater treatment plants.
Currently, some research is attempting to recover N2O from wastewater. Coupled aerobic–anoxic nitrous decomposition operation (CANDO) induces heterotrophic denitrifying bacteria to store carbon sources in the form of polyhydroxyalkanoates (PHAs) in cells by alternately providing carbon and nitrogen sources in pulses. PHA does not easily provide electrons, resulting in the accumulation of N2O, with a conversion rate of up to 87% [11]. A photocatalytic autotrophic denitrification system based on the combination of semiconductor CdS and Thiobacillus denitrificans can also achieve efficient conversion of N2O. By utilizing the characteristic that sulfides can react with metal active centers, the activity of nitrous oxide reductase (Nos) was inhibited, resulting in a high N2O conversion efficiency of 71% to 73.2% [12]. In a sulfur autotrophic denitrifying bacteria-dominated reactor, the NO, which was fixed by Fe(II)EDTA as the electron acceptor for denitrification, can inhibit the activity of Nos within an appropriate threshold, and achieved a 41% conversion of N2O [13]. However, the above processes will also inhibit the activity of other enzymes while regulating the nitrogen converted to N2O and limiting the efficiency of nitrogen removal. This defect will be further magnified when dealing with complex wastewater in practice. In a previous study, a denitrifying strain with high N2O conversion performance, Pseudomonas aeurginosa PAO1, was constructed by knocking out nosZ (gene coding for Nos) [14]. With the nosZ-deficient strain of P. aeurginosa (ΔnosZ P. aeurginosa) supplied into the incineration leachate nitrogen removal process for microflora optimization, denitrification efficiency of 99% was achieved, and the N2O conversion efficiency was as high as 95% [15]. The above results demonstrate the ability of ΔnosZ P. aeurginosa to deal with complex wastewater.
However, in order to efficiently convert N2O by supplying ΔnosZ P. aeruginosa in the denitrification process, it is necessary to strengthen the dominant position of this functional bacterium. During long-term operation in a moving bed biofilm reactor (MBBR), the abundance of ΔnosZ P. aeruginosa was observed to decrease to 38% [15]. Subsequent research has found that using an MEC to stimulate ΔnosZ P. aeruginosa to secrete more phenazine derivatives can greatly improve its denitrification ability, biofilm formation rate, and survival advantage through the electron transfer ability, signaling molecule function, and antibacterial ability of phenazine derivatives [16]. Its abundance can be maintained at 66% in the ΔnosZ P. aeruginosa/MEC system. The process of indirect electron transfer between phenazine derivatives and electrodes by ΔnosZ P. aeruginosa may be a key factor in affecting its dominant position in the ΔnosZ P. aeruginosa/MEC system. Recent studies showed that phenazine could enhance the electron transport between P. aeruginosa PAO1 and a poised-potential electrode under anaerobic conditions [17]. Different electrode surfaces also influence phenazine production when P. aeruginosa requires electrode respiration [18]. However, few studies focus on the relationship between electroactivity and denitrification of P. aeruginosa. In a previous study, the electroactivity of P. aeruginosa was enhanced by supplying a quorum-sensing signal molecule, which resulted in a higher nitrogen removal efficiency and more stable N2O conversion efficiency [19]. This method allows P. aeruginosa to generate more phenazines to transfer electrons, but the role of the electrode as the electron receiver is ignored. Therefore, the next step to improve the sustainability of this system for recovering N2O is reveal the effect of electrode materials on the denitrification of ΔnosZ P. aeurginosa.
In this study, a carbon felt electrode and graphite electrode were set up in two ΔnosZ P. aeurginosa/MEC systems to investigate the effect of electrode materials on nitrogen conversion performance, gene expression involved in related processes, and the bacterial community structure in the ΔnosZ P. aeurginosa/MEC system. The goal is to further strengthen the dominant position of ΔnosZ P. aeurginosa from the perspective of electrode material optimization and achieve more efficient conversion of N2O from incineration leachate.

2. Materials and Methods

2.1. Configuration of the ΔnosZ P. Aeruginosa/MEC System

The bioreactors used in this study were based on a two-chamber MEC construction (Figure 1). Each chamber of the reactors contains a liquid volume of 250 mL and a headspace volume of 150 mL. The working electrodes equipped in the anode chamber and the counter electrodes equipped in the cathode chamber were adopted with the same size of 60 × 30 × 5 mm whether formed with graphite material or carbon felt material. The specific surface area of graphite material and carbon felt material was 1.82 m2/g and 2.34 m2/g, respectively. Both the electrode materials were connected with a nickel rod before being applied into the microbial electrochemical system. Additionally, Ag/AgCl electrodes (+199 mv vs. SHE) saturated with KCl were also supplied in the anode chamber and reserved as the reference electrode. The anode potential was controlled using a potentiostat (Chil1030C, Shanghai, China). The anode and cathode chambers were separated by a 9.6 cm2 anion exchange membrane (AEM, ASTOM Co., Tokyo, Japan).
The ΔnosZ P. aeruginosa strain was enriched in LB medium for 24 h at 37 °C and subsequently inoculated into a laboratory-scale MBBR for biofilm formation until the biomass attached to the polypropylene fillers (embedded with a cross-shaped carrier) reached 0.31 ± 0.02 mg per filler. Each anode chamber of the MEC reactors was supplied with 30 fillers immobilized with ΔnosZ P. aeruginosa. The raw incineration leachate was collected from an MSW energy incineration plant in Beijing and sterilized using a 0.22 μm filter to remove any microbes present in the leachate. The pre-treated incineration leachate was then supplied to the anode chamber as a carbon source for denitrification by ΔnosZ P. aeruginosa. The cathode chamber was continuously fed with partially nitrification-treated leachate, and nitrite that was able to pass through the AEM served as an electron acceptor for denitrification in the anode chamber. Characteristics of experimental wastewater used in this study are summarized in Table 1.

2.2. Experimental Set-Up

Two ΔnosZ P. aeruginosa/MEC systems with carbon felt electrodes and graphite electrodes were used in this study. Both experimental groups were used under the same manner. The operation temperature was maintained in the range of 30 ± 1 °C. The anode potential applied in this study was +0.8 V. The concentration of NO2−-N in partial nitrification-treated leachate was ~1000 mg/L when it served as the inflow of the cathode chamber.
In the process of the experiment, the hydraulic retention time (HRT) was gradually shortened, and it depended on the NO2−-N removal efficiency of ΔnosZ P. aeruginosa of each group. The NO2−-N concentrations of the anode chamber and cathode chamber in each group were monitored daily. The NO2−-N removal efficiency, which reflected the overall nitrite removal performance of the ΔnosZ P. aeruginosa/MEC system, and the N2O conversion efficiency, which reflected the N2O conversion capabilities of the ΔnosZ P. aeruginosa, were calculated with the equation described in a previous study [16].

2.3. RNA Extraction and Quantitative Reverse Transcription PCR (qRT-PCR)

Microbial cells in a suspended state and the anode electrode were harvested on day 21 and day 45, respectively. A centrifuge operating at 8000 rpm for 10 min was used to concentrate the cells, which were then used for RNA extraction using the RNAprep Bacteria Kit (Aidlab Biotechnologies, China). To ensure RNA purity, an additional step was taken to remove residual DNA using DNA-free DNase (New England Biolabs, Beijing, China) following the manufacturer’s instructions.
Purified RNA (1 μg) was used to synthesize cDNA through reverse transcription using the THERMOscript 1st Strand cDNA Synthesis Kit (Aidlab Biotechnologies, Beijing, China) according to the manufacturer’s instructions. The resulting cDNA products were used as templates for quantitative polymerase chain reaction (qPCR) with primers targeting genes involved in denitrification and phenazine biosynthesis, as well as the constitutively expressed housekeeping gene proC. qPCR detection was performed using a real-time PCR system (7500 FAST, USA), as previously described [15,16,19].

2.4. DNA Extraction and Sequence Analysis

On days 21 and 45, microbial cells growing in the anode chamber of the ΔnosZ P. aeruginosa/MEC system were harvested for DNA extraction using the RNeasy PowerSoil DNA elution kit (QIAGEN), following the manufacturer’s instructions. The concentration and purity of the extracted DNA samples were determined using the Nanodrop UV spectrophotometer (Thermo Fisher Scientific, Delaware).
Bacterial 16S rRNA gene fragments were amplified via PCR using the 338F/806R primer set. The resulting amplicons were sequenced on an Illumina Hiseq 2000 platform (Illumina, San Diego, CA, USA) by Majorbio Bio-Pharm Technology Co., Ltd. (Shanghai, China). The sequences were then sorted into various operational taxonomic units using Pyrosequencing Pipeline software (https://pyro.cme.msu.edu) accessed on 3 July 2021. The raw sequence files have been deposited in the NCBI Sequence Read Archive database under accession NO. PRJNA974214.

2.5. Analytical Methods

In this study, the water quality index, including NO2−-N and COD concentrations, was determined using standard methods (APHA, 2005). The measurement of N2O followed a previous study [16]. To measure the biomass in the middle of a continuous experiment, two parallel reactors of carbon felt and graphite were used. When measuring the biomass, the biomass of all states was collected from the parallel reactors. The immobilized biomass and the biomass on the electrode were measured for dry weight after scraping or squeezing until cells were no longer obtained from the carriers. The biomass in suspended state was measured through the linear relation between OD600 and biomass [19]. Phenazine derivatives in the anode chamber were extracted using chloroform and then re-extracted using deionized water with different pH levels. The concentration of phenazine-1-carboxylic acid (PCA), 1-hydrophenazine(1-OH-PHZ), and pyocyanin (PYO) were determined using the linear relation between the standard substance and absorbency under 252 nm (neutral), 520 nm (alkaline), and 520 nm (acidic), respectively [20,21].

3. Results

3.1. Performance of ΔnosZ P. aeruginosa/MEC Reactors with Carbon Felt Electrodes and Graphite Electrodes

The ΔnosZ P. aeruginosa/MEC systems with carbon felt electrodes and graphite electrodes were operated for 45 days, and the influent NO2−-N concentration of partial nitrification-treated incineration leachate was about 1000 mg/L (Figure 2). From day 1 to day 9, the NO2−-N removal efficiency of two groups of reactors gradually increased to 83.3% and 78.1%, respectively, through the enrichment of ΔnosZ P. aeruginosa in the anode chamber. In the subsequent stage (from day 9 to day 33), the HRT was reduced to 6 days, and the average cathode NO2−-N effluent concentrations of the carbon felt group and graphite group reactors were 183.3 mg/L and 234.9 mg/L, respectively. The carbon felt group exhibited an average TN removal rate of 81.92%, which was 6.63% higher than that of the graphite group (Figure 2). On day 33, the HRT was further shortened to 3 days. At this point, the cathode NO2−-N effluent concentration of the two groups of MEC reactors increased due to the mass transfer capacity of the anion exchange membrane reaching its upper limit, with average values of 239.3 mg/L and 294.4 mg/L, respectively. The average NO2−-N removal rate decreased to 76.22% and 69.66%, but the carbon felt group still exhibited a 6.56% higher NO2−-N removal rate than the graphite group (Figure 2).
Based on the N2O conversion performance of the ΔnosZ P. aeruginosa/MEC system with two different electrode materials (Figure 3), there was no significant difference in gaseous N2O production (4.05 ± 0.04 mmol) between the carbon felt and graphite groups from day 1 to day 21. However, the N2O conversion efficiency of the carbon felt group was 5.18% lower than that of the graphite group, indicating that the N2O conversion process by ΔnosZ P. aeruginosa in the carbon felt group reactor was interfered with during this stage. Subsequently, after day 21, the N2O conversion performance of the carbon felt group reactor gradually surpassed that of the graphite group. The average gaseous N2O production was higher in the carbon felt group than in the graphite group by 0.51 ± 0.02 mmol, with the highest difference observed on day 45 (0.83 ± 0.03 mmol). Additionally, the N2O in the carbon felt group remained above 80%, while there was a downward trend in the graphite plate group after day 39 of operation (Figure 3). The proportion of N2O in biogas production represents its recovery potential, and on day 45, N2O accounted for 80.6% of biogas production in the carbon felt group, which was 16.8% higher than that in the graphite group (Figure 3).
The above findings suggest that the carbon felt may be a more effective anode electrode material for the ΔnosZ P. aeruginosa/MEC system in terms of N2O conversion performance and recovery potential. Further research is needed to investigate the underlying mechanisms behind these observations.

3.2. Effect of Electrode Materials on Denitrification, Electroactivity, and Biomass of ΔnosZ P. aeruginosa

qRT-PCR analysis was performed for expression of denitrification and electroactivity genes in ΔnosZ P. aeruginosa/MEC systems with carbon felt electrodes and graphite electrodes (Figure 4). The transcription of the norB gene of the terminal nitric oxide reductase of ΔnosZ P. aeruginosa is shown in Figure 4a. Gene expression of norB in both groups of reactors increased on day 45 compared to day 21, indicating that electric stimulation gradually enhanced the effect on ΔnosZ P. aeruginosa. By comparing the gene expression of suspended bacteria, it can be observed that the difference multiple of norB expression between the carbon felt group and the graphite group gradually decreased from 3.08 times on day 21 to 1.11 times on day 45 (Figure 4a). The expression levels of phzG, phzM, and phzH involved in regulating phenazine derivative synthesis in suspended bacteria in the carbon felt group decreased on day 45 compared to day 21, but the opposite was true for the graphite group. The biomass results indicate that the gap between the biomass of suspended bacteria in both groups increased on day 45, with 74.83 ± 2.21 mg and 56.28 ± 2.54 mg for the carbon felt group and graphite group, respectively (Figure 5a). On day 45, the value of total phenazine derivative per unit biomass accessible by the carbon felt group (0.096 ± 0.002) was not significantly different from that of the graphite group (0.085 ± 0.006) (Figure 5b). The reason for this result may be that the biomass attached to the surface of the carbon felt electrode is higher (2.42 times that of the graphite group), which excessively occupies the opportunity for suspended bacteria to use phenazine derivatives for electron transfer with electrodes. The advantage of the carbon felt group over the graphite group for suspended bacteria lies mainly in higher synthesis of phenazine in early operation, which promotes biomass increase.
On day 45 of operation, the norB gene expression of electrode-attached and suspended ΔnosZ P. aeruginosa was compared. The norB gene expression of ΔnosZ P. aeruginosa on the carbon felt electrode was found to be much higher than that of the suspended state (2.73 times), as shown in Figure 4a. Additionally, the gene expression levels of phzG, phzM, and phzH of ΔnosZ P. aeruginosa attached to the carbon felt electrode were 7.64 times, 10.61 times, and 2.59 times that of the graphite plate group, respectively (Figure 4b). The total phenazine derivative per unit biomass accessible by the carbon felt group was more than twice that of the graphite plate group. These results suggest that the carbon felt electrode is more effective than the graphite electrode in promoting the denitrification process and electrode respiration of ΔnosZ P. aeruginosa.

3.3. Bacterial Community Analysis

The bacterial community structure enriched under the influence of two different electrode materials is analyzed in Figure 6. On day 21 of operation, the abundance of the Pseudomonas genus in the carbon felt group was high, at 91.7%, while that in the graphite group was 82.9%. Furthermore, it was observed that more genera appeared in the carbon felt group, such as Alkaliphilus (4.57%), Eubacterium (2.94%), and Citrobacter (0.76%). Alkaliphilus can convert nitrite nitrogen in the system into ammonia nitrogen through dissimilatory nitrate reduction [22] and has been found to be enriched in heterotrophic denitrifying microbial fuel cells [23], but it does not have denitrifying function. Therefore, the decrease in N2O conversion rate in the carbon felt group in Figure 3 is due to Alkaliphilus in the community converting part of the nitrite into ammonia nitrogen. Eubacterium does not have any genes related to denitrification, but it is related to AI-2 type signal molecule-induced quorum sensing [24,25], which may be related to the lux quorum-sensing system regulated by the luxS gene carried by Eubacterium [26]. This has a synergistic effect on quorum sensing with the Pseudomonas genus.
On day 45 of operation, the abundance of Pseudomonas genus in the carbon felt group and graphite group was 82.6% and 73.8%, respectively, both decreased compared to day 21. The abundance of Pseudomonas genus in the carbon felt group remained relatively high, and it was found that the Alkaliphilus and Citrobacter originally present in the community disappeared, indicating that these invading genera gradually became unsuitable for the environment within the MEC reactor with its operation, which may be related to the antibacterial effect of some secondary metabolites secreted by Pseudomonas. In addition, Eubacterium still exists as a heterotrophic bacterium producing N2O from denitrification in both MEC reactors, indicating that there is indeed a synergistic symbiotic relationship between Eubacterium and Pseudomonas genus.

4. Discussion

The ΔnosZ P. aeruginosa/MEC system with carbon felt electrodes has shown 5~6% improvement of nitrite removal and N2O conversion efficiencies compare to the one equipped with graphite electrodes. In this system, as described in Figure 1, nitrite in partial nitrification-treated leachate needs to pass through the AEM and then be utilized by the ΔnosZ P. aeruginosa as the electron acceptor for denitrification. There are two ways to impact the nitrite consumption, that is, the electrode attraction by the anode and the denitrification ability of ΔnosZ P. aeruginosa. However, a previous study confirmed that the electrode attraction showed little effect on nitrite transfer from the cathode chamber to the anode chamber. Therefore, the denitrification ability of ΔnosZ P. aeruginosa is the major approach to illustrate the improvement by carbon felt electrodes. In the denitrification pathway of ΔnosZ P. aeruginosa, norB is the terminal gene which regulates the NO converted to N2O. qRT-PCR results of the norB can represent the denitrification ability of the individual ΔnosZ P. aeruginosa cell and showed a high expression in the carbon felt group at the early operation phase (day 21, biomass in suspended state) and the end of operation (day 45, biomass attached on electrode). It revealed that the carbon felt may better enhance the denitrification of the ΔnosZ P. aeruginosa cell by establishing a more efficient electron transfer chain. Otherwise, the cells with higher expression of norB will not be found in the conditions that possess the maximum chance of contact with the electrode. Meanwhile, this could easily imply that the nitrogen removal performance not only depends on the ability of individual cells but also relates to the biomass quantity of ΔnosZ P. aeruginosa. Fortunately, the biofilm formation of ΔnosZ P. aeruginosa has a deep connection with a type of signal molecule, called phenazine derivates [27,28]. Phenazine derivates also play an important part in the indirect electron transfer between the ΔnosZ P. aeruginosa cells and electrode [16,17,29,30]. The higher concentrations of phenazine derivates and much more expressed genes of phenazine derivates synthesis were also found in the same conditions (day 21, biomass in suspended state and day 45, biomass attached on electrode), in accordance with norB expression and the biomass in the carbon felt group. The results showed a strong correlation among the denitrification, electroactivity, and biomass in ΔnosZ P. aeruginosa. Phenazine derivates are key compounds of the two electrode materials and showed differences in the coupled systems.
The improvement of N2O conversion by using carbon felt electrodes in the ΔnosZ P. aeruginosa/MEC system is the major finding for achieving the long-term recovery of N2O from incineration leachate. A total of 82.6% of the Pseudomonas genus survived, with the other genus eliminated in the carbon felt group. It may relate to the antibiotic characteristics of phenazine derivates or the gradually enhanced advantages during the operation process [31,32,33,34]. In conclusion, multiple results support the fact that carbon felt can optimize the ΔnosZ P. aeruginosa/MEC system predictably. Further study needs to explore the microbial electrochemical characteristics when operating the ΔnosZ P. aeruginosa/MEC system with the carbon felt electrode.

5. Conclusions

In this study, two ΔnosZ P. aeruginosa /MEC systems equipped with carbon felt electrodes and graphite electrodes were operated to analyze the optimization mechanism of electrode materials for N2O recovery during incineration leachate treatment. The carbon felt group showed better performance in N2O recovery during the 45-day operation, with N2O conversion efficiency above 80% and the proportion of N2O in biogas accounting for 80.6%. qRT-PCR analysis was conducted to evaluate the expression of genes involved in denitrification (norB) and electroactivity (phzG, phzM, and phzH) of ΔnosZ P. aeruginosa. The results showed significant upregulation in the suspended biomass (day 21) and the electron-attached biomass (day 45) from the carbon felt-equipped reactor, which was highly related to the opportunity of biomass exposed to the phenazine derivatives. The carbon felt group also had a higher survival rate of 82.6% of the Pseudomonas genus after 45 days of operation (8.8% higher than the graphite group). These results indicate that the carbon felt electrode has a more sustainable performance for N2O recovery in the ΔnosZ P. aeruginosa/MEC system.

Author Contributions

Conceptualization, D.S. and H.N.; methodology, H.N.; software, S.G.; validation, H.J.; formal analysis, H.J.; investigation, Y.L.; resources, H.N. and D.S.; data curation, Y.L.; writing—original draft preparation, Y.L. and H.N.; writing—review and editing, B.Y. and S.X.; visualization, Y.L.; supervision, D.S. and H.N.; project administration, D.S. and H.N.; funding acquisition, D.S., B.Y. and S.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 51678051; the Jiangxi Provincial Natural Science Foundation, grant number 20212BAB213042; Excellent Youth Training Program of Jiangxi Academy of Sciences, grant number 2022YSBG50006 and Introduction of Doctor Program of Jiangxi Academy of Sciences, grant number 2023YYB02.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of the ΔnosZ P. aeruginosa/MEC system.
Figure 1. Schematic diagram of the ΔnosZ P. aeruginosa/MEC system.
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Figure 2. Nitrite removal of ΔnosZ P. aeruginosa/MEC systems with carbon felt electrodes and graphite electrodes.
Figure 2. Nitrite removal of ΔnosZ P. aeruginosa/MEC systems with carbon felt electrodes and graphite electrodes.
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Figure 3. Nitrous oxide conversion of ΔnosZ P. aeruginosa/MEC systems with carbon felt electrodes and graphite electrodes. (a) N2O production; (b) N2O conversion efficiency; (c) N2O proportion in biogas. Error bars represent standard deviations of triplicate measurements.
Figure 3. Nitrous oxide conversion of ΔnosZ P. aeruginosa/MEC systems with carbon felt electrodes and graphite electrodes. (a) N2O production; (b) N2O conversion efficiency; (c) N2O proportion in biogas. Error bars represent standard deviations of triplicate measurements.
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Figure 4. Denitrification- and electroactivity-related gene expression in the ΔnosZ P. aeruginosa/MEC systems with carbon felt electrodes and graphite electrodes on day 21 and 45. (a) norB; (b) phzG, phzM, and phzH. Error bars represent standard deviations of triplicate measurements.
Figure 4. Denitrification- and electroactivity-related gene expression in the ΔnosZ P. aeruginosa/MEC systems with carbon felt electrodes and graphite electrodes on day 21 and 45. (a) norB; (b) phzG, phzM, and phzH. Error bars represent standard deviations of triplicate measurements.
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Figure 5. Biomass and the opportunity of touching phenazine derivatives per unit of biomass in the ΔnosZ P. aeruginosa/MEC systems with carbon felt electrodes and graphite electrodes on day 21 and 45. (a) Biomass; (b) phenazine derivative content/biomass. Error bars represent standard deviations of triplicate measurements.
Figure 5. Biomass and the opportunity of touching phenazine derivatives per unit of biomass in the ΔnosZ P. aeruginosa/MEC systems with carbon felt electrodes and graphite electrodes on day 21 and 45. (a) Biomass; (b) phenazine derivative content/biomass. Error bars represent standard deviations of triplicate measurements.
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Figure 6. Bacterial community analysis on anode of the ΔnosZ P. aeruginosa/MEC systems on day 21 and 45.
Figure 6. Bacterial community analysis on anode of the ΔnosZ P. aeruginosa/MEC systems on day 21 and 45.
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Table
Table 1. Characteristics of experimental wastewater used in this study.
Table 1. Characteristics of experimental wastewater used in this study.
ItemCOD
(mg/L)		BOD5
(mg/L)		NH4+-N
(mg/L)		NO3−-N (mg/L)NO2−-N (mg/L)pH
Raw leachate57,686–75,48023,858–33,710968 ± 1368N/AN/A6.01–6.37
Anaerobically
treated leachate		1820–4625879–31821029–1458N/AN/A7.95–8.45
Partial nitrification-treated leachate1524–212397–334178–20421.78–35.53945–11257.27–7.93
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MDPI and ACS Style

Liu, Y.; Yan, B.; Xia, S.; Gui, S.; Jiang, H.; Nie, H.; Sun, D. Electrode Material Optimization of Nitrous Oxide Recovery from Incineration Leachate in a ΔnosZ Pseudomonas aeruginosa/MEC System. Fermentation 2023, 9, 607. https://doi.org/10.3390/fermentation9070607

AMA Style

Liu Y, Yan B, Xia S, Gui S, Jiang H, Nie H, Sun D. Electrode Material Optimization of Nitrous Oxide Recovery from Incineration Leachate in a ΔnosZ Pseudomonas aeruginosa/MEC System. Fermentation. 2023; 9(7):607. https://doi.org/10.3390/fermentation9070607

Chicago/Turabian Style

Liu, Yong, Bing Yan, Song Xia, Shuanglin Gui, Haiwei Jiang, Hanbing Nie, and Dezhi Sun. 2023. "Electrode Material Optimization of Nitrous Oxide Recovery from Incineration Leachate in a ΔnosZ Pseudomonas aeruginosa/MEC System" Fermentation 9, no. 7: 607. https://doi.org/10.3390/fermentation9070607

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